The lithographic (offset) printing method for multi-color prints using the so-called color-separation technique generally comprises photographing the color original to be reproduced through color filters for obtainment of continuous-tone negatives, i.e. the so-called separation negatives. As color original, a color transparency is usually employed which is separated on a conventional film for separation negatives with a range of 1.6-1.8 and with a sensibility of e.g. 40-50 ASA, and from this separation negative a screen positive is made on a lith film. Screen-free color-separation negatives (continuous-tone negatives) prepared in the above-mentioned manner are then illuminated through a screen and directly onto a new film.
However, the screen may also be introduced into the system by illuminating the color original through the color filter and a screen and onto a negative film ("direct screening") thereby obtaining a color-separation negative with screen pattern (halftone negatives) which will be found in the printed copy of the original.
In multi-color printing a printing form is produced for each color to be printed. In three-color printing the printing inks used are yellow, cyan and magenta, however, usually it is printed in four colors with black as correction color, and the colors are usually, but not necessarily, printed in this sequence.
Thus, in the conventional lithographic (offset) multi-color printing process the printing form is prepared and developed with a photographed screen pattern, and this screen pattern will be transferred to the printed copy of the original and constitute a foreign element therein with regard to the original. However, there are often problems with the screen method, particularly when used for high speed printing of the image as this may easily become blurred. For conventional four-color auto-illustrations for newspapers and magazines printed in letter-press there must also technical reasons be used a relatively coarse screen, e.g. having from 20 to 24 lines per linear cm.
A well-known basic method for reproducing color originals and which involves preparing three color-separation negative screens corresponding to the three primary colors, and in certain cases also a black correction negative, is disclosed e.g. in U.S. Pat. Nos. 1,843,872 and 1,885,725.
When gravure cylinders for multi-color printing are to be produced it is also known to prepare color-separation positives from the corresponding color-separation negatives. When separation negatives obtained by photographing the original through a red filter are used for the preparation of the separation positive the resultant product is a cyan color-separation positive. The separation negative obtained by photographing the color original through a green filter will result in a magenta separation positive, and the separation negative obtained by photographing the color original through a blue filter will result in a yellow separation positive. There is also frequently prepared a black color-correction positive because even the highest qualities cyan, magenta and yellow printing inks do not combine to form a neutral black as theory would dictate. The correction positive will make up for this tonal irregularity in practice, so that when the three colors are combined, a neutral black is ultimately achieved. Reference may in this connection be made to British patent specification No. 1,136,464.
The four color-separation positives are then used for preparing the gravure printing plates to be used for the multi-color printing. This preparation of gravure printing plates is well known within the art and will here not be further explained, however, reference can be made to U.S. Pat. No. 3,179,519.
Throughout the world the printing processes, relief (flexography, letterpress), planographic (offset), intaglio (gravure) and porous (screen process) are in daily use. As regards the reproduction of tonal values there is a substantial difference between the intaglio (gravure) process and the other printing processes although they have all hitherto used a so-called screen.
In gravure printing the function of the intaglio screen is to divide the printing surface into so-called steps which support the doctor blade, i.e., the knife which scrapes the printing ink from the surface of the printing form, and into wells which contain the printing ink. The wells are etched into the printing form, and it is the depth of the etched well which determines the amount of printing ink and thereby the thickness of the printing ink layer transferred to the paper. That the concentration of the color pigments varies over the printing surface and results in variable light absorption is characteristic of what is termed a genuine half-tone. The other printing processes may only transfer the color layer with uniform thickness over the entire printing surface. In order to obtain different tonal values (varying light absorption) with these other printing processes the printing surface must be divided into very small printing and non-printing areas so that to the eye these are mixed into tonal values.
This division is usually done with a so-called autotypic or halftone screen. There are many different types and techniques; however, it may be generally stated that the screen divides the printed image into large and small dots related to the tonal values of the original. All dots are equally spaced, however, the number thereof per cm.sup.2 depends upon the fineness of the screen. The size of the dot is stated in percent of the completely covered surface and the fineness in number of lines per cm. The finer the screen, the better the reproduction of details for a certain size; however, the screen must be selected with due consideration to printing technique and paper quality. Herein resides some of the limitation of the screen technique, i.e. that sharpness and reproduction of details are dependent upon the printing technique and the paper quality.
If the printed colors of a multi-color illustration do not fall exactly on top of one another, a blurred reproduction is produced. This is termed imperfect register. A halftone screen is like a cross-line grid and forms screen dots along straight lines at an angle of exactly 90.degree. with each other.
When printing in black and white this crossline grid is positioned at an angle of 45.degree. with regard to the eyes of the reader and is almost imperceptible. If two such crossline grids are printed above one another this may result in a noticeable frame formation which is called moire pattern. The smaller the angle between the lines, the more noticeable the moire pattern and vice versa. At an angle of 30.degree., the effect is not visible; however, in four-color printing it is impossible to print all the colors at 30.degree. angles. Two colors must, accordingly, be positioned at the same angle or some colors at an angle of only 15.degree. or at an angle of 22.5.degree., and some moire pattern must be accepted in certain cases.
With intaglio printing the ratio between line and well is usually equal to 2:5, and with a sucking printing ink the screen pattern more or less merges so that screen angles are not a matter of concern. The same advantage is obtained by the screen-less color printing technique with which the present invention is concerned.